feat: Go eval support

[David Elner (delner)]

What

Adds Go to EvalLanguage, so bt eval can run Go evals.

Why

The Braintrust Go SDK now implements the runner protocol (evalrunner package),
but bt could not dispatch to it: EvalLanguage was a closed
{JavaScript, Python} enum and detect_eval_language hard-errored on any other
extension. This is the companion change.

How

Go needs a shape bt did not have. For JS and Python, bt supplies the
program — its own embedded eval-runner.{ts,py} — and the user supplies files
that program imports at runtime. Go has no runtime loading: the user's
compiled package is the runner. So bt simply
executes what the user built. Go's compilation unit is also a directory, not a
file, so all inputs must resolve to one package.

bt eval --language go ./cmd/evals            # compile and run the package
bt eval ./cmd/evals                          # inferred from a *_eval.go marker
bt eval --language go --runner ./bin/evals   # run a prebuilt binary
bt eval --dev ./cmd/evals                    # dev server for the playground

• Discovery: **/*_eval.go, a suffix mirroring Go's own _test.go
  convention. The matched file is only a marker — go run compiles the whole
  package. Pass --language go for a package with no such file.
• Command: go run <package dir>. With --runner, the binary is spawned
  with no arguments — the Go runner takes all input from BT_EVAL_* and
  ignores argv, and appending files would break a main that parses flags.
• Toolchain: BT_EVAL_GO_BIN / BT_EVAL_GO, then GOROOT, then PATH.

Everything else — env vars, the SSE socket, the stdout manifest, drive_eval_runner —
is language-agnostic and unchanged.

Other behavior changed

Worth calling out, since these are not Go-specific:

1. prepare_eval_runners is now lazy. It was writing both
   eval-runner.ts and eval-runner.py to disk on every spawn, before bt knew
   which language it was running — and under --dev that is once per HTTP
   request. It now runs inside the language arms that need it.
2. vendor added to DEFAULT_EXCLUDE_DIRS, so bare bt eval in a Go repo
   does not pick up a vendored dependency's eval files. This also applies to
   JS/Python discovery, but only in the zero-argument case.
3. **/*_eval.go added to DEFAULT_EVAL_GLOBS. A polyglot repo containing
   both TS evals and a *_eval.go file will now get "Mixed eval file types are
   not supported yet" from bare bt eval — an existing failure class, new
   trigger.
4. expand_eval_file_globs now takes the --language override, because it
   runs before detect_eval_language and would otherwise reject a Go package
   directory before the override was ever consulted.

Watch mode

A Go binary cannot report its own sources, so the watch set is derived
statically: the package directory (whose mtime changes when files are added or
removed) plus its buildable .go files. Imports in other packages are not
followed.

Tests

17 new tests. detect_eval_language had no coverage at all before this, so Go
adds the first. Verified end to end against the Go SDK: /list, a streaming
playground run, and a CLI bt eval run.

[David Elner (delner)]

Just of note: this includes support for both source and precompiled Go runners. If we prefer to simplify to one path, we could do that instead.

[github-actions]

Latest downloadable build artifacts for this PR commit 7cdbb8da7e01:

• Workflow run: https://github.com/braintrustdata/bt/actions/runs/31753197300
• Download all artifacts (GitHub CLI): gh run download 31753197300 --repo braintrustdata/bt
• Installers are published from main automatically. To publish one for a PR branch, run release-canary manually via workflow_dispatch.

Available artifact names

• artifacts-build-global
• artifacts-build-local-aarch64-pc-windows-msvc
• artifacts-build-local-x86_64-pc-windows-msvc
• artifacts-build-local-x86_64-apple-darwin
• artifacts-build-local-x86_64-unknown-linux-musl
• artifacts-build-local-aarch64-apple-darwin
• artifacts-build-local-x86_64-unknown-linux-gnu
• artifacts-build-local-aarch64-unknown-linux-gnu
• artifacts-plan-dist-manifest
• cargo-dist-cache